In recent years, in various fields, there have been growing concerns about environmental problems.
With these concerns, in the field of electric power supply, certain approaches to power supply such as photovoltaic (PV) power generation and use of secondary cells used in electric vehicles (EVs) and hybrid EVs (HEVs) have attracted attention. Lithium-ion secondary cells are considered promising in such secondary cells. It is expected that lead storage cells will be replaced with Lithium-ion secondary cells in response to widespread use thereof in the future.
As a rule, connecting an old cell (having a small discharge capacity, a high degree of degradation) and a new cell (having a larger discharge capacity, a low degree of degradation) in parallel with each other should be avoided. This is for the purpose of avoiding a cross current caused by a difference between the voltages of the cells. A large cross current can cause excess current or abnormal heat generation. The same can be said with respect to the above-described secondary cells.
In order to adjust the degrees of degradation of a plurality of cells connected in parallel with each other, a system has therefore been devised in which respective switches for making connection/disconnection are provided for the plurality of cells, and a switch that is provided only for the cell that has a higher voltage than that of the other cells is connected (maintained in the on state) at the time of discharge until the differences between the voltages of the cells become equal to or smaller than a predetermined value (see, for example, Patent Literature 1).